Class II nonmuscle myosins are implicated in participating in diverse biological processes such as cytokinesis, cellularization, cell shape changes and gastrulation. Two distinct nonmuscle myosin heavy chain genes have been reported in all vertebrates: nonmuscle myosin heavy chain-A (NMHC-A) and -B (NMHC-B). We are working on the isolation of the Xenopus homolog of NMHC-A and are carrying out a comparative analysis of the developmental and spatial expression patterns of NMHC-A and the previously isolated NMHC-B to address the role of NMHCs in vertebrate development. A 7.5 kb NMHC-A mRNA is present maternally in unfertilized eggs and throughout embryogenesis as well as in all adult tissues examined. An additional 8.3 kb zygotic transcript for NMHC-A is first detectable at stage 15, peaks between stages 25-30 and is greatly diminished by stage 47. Whole mount in situ hybridization with tailbud stage embryos shows that NMHC-A mRNA is predominantly expressed in the epidermis, whereas NMHC-B mRNA is expressed in the somites, brain, eyes and branchial arches. Interestingly, the expression of NMHC-B in developing somites is gradually restricted to the center of each somite as differentiation proceeds, forming an expression pattern of vertical lines at the tailbud stage. DAPI nuclear staining of sections from NMHC-B stained embryos demonstrated that NMHC-B mRNA is colocalized with the nuclei and/or perinuclear area. In animal cap experiments, treatment with activin A up-regulates NMHC-B as well as muscle actin mRNAs and down-regulates NMHC-A mRNA, consistent with NMHC- B expression in the somitic muscle and NMHC-A expression in the epidermis. Thus, NMHC-A and NMHC-B mRNAs are differentially expressed in Xenopus development and the nuclear localization of NMHC-B mRNA suggests a unique function of this gene during somitogenesis.